The proposed research is devoted to testing a specific model for the replication of chromatin and the inheritance of a pattern of nucleosomic domains along the chromatid. These domains are composed of some 20-200 chemically distinct nucleosomes of a uniform type that are wound into a helical superstructure to produce the 250 A chromatin thread. The pattern of these domains along the chromatid determine the pattern of possible transcription into nuclear RNA, and thereby the potential pattern of gene expression. The model is based on the known features of chromatin structure and replication. We expect nucleosomes to be chemically heterogeneous and composed of histones of various sequences, and further that nucleosomes of a given domain are all identical. The model shows that the location of boundaries between domains cannot be replicated precisely and they are obliged to wander randomly. Thus, cells of greater replicative age may express genes abnormally. Our first project is to determine whether nucleosomes of a common type are clustered. We intend to do this by separating nucleosomes on gels and testing to see whether distinct electrophoretic classes contain DNA sequences that are homologous to a cloned piece of genomic DNA. Our second project is aimed at the purification of nucleosomes from active chromatin and analyzing the sequences of the histones they contain. This can be done by peptide analysis following Staphylococcus protease digestion. The third project will test whether homologous nucleosomes specifically associate with one another by sedimentation and autoradiographic experiments. The fourth project will probe the replication of a physical domain that contains the 5S genes. The fifth project is a biological test of the domain model. If genes for specific histones are deleted genetically, then certain classes of nucleosomes will be unavailable. This will cause stage specific arrest of the developmental program. We intend to relate this stage specific arrest with the chemical type of nucleosomes (histones) that are missing.